This invention relates to improvements in a circuit for controlling the scanning rate of an ion beam in an ion implantation system.
In recent years, in the field of the manufacture of semiconductor devices, the ion implantation method has been introduced for the doping of a semiconductor substrate with an impurity instead of the conventional thermal diffusion method.
The ion implantation method is such that an impurity, for example, phosphorus or boron is efficiently ionized, that the impurity ions are accelerated in a high vacuum vessel with a high voltage, and that a silicon substrate (or wafer) is irradiated and implanted with the accelerated ions.
As disclosed in U.S. Pat. No. 3,778,626, an ion implantation system suitable for the mass production carries out the ion implantation by rotating a disc on which a large number of wafers are placed along a circumference. The ion implantation of the wafers proceeds while rotating the disc at a constant speed about the axis of rotation thereof and scanning an ion beam radially of the disc. In this case, as the ion beam scans towards the axis of rotation of the disc, the linear velocity of the ion beam relative to the disc lowers. This results in the problem that the quantity of the ions implanted in the wafer is larger in the central part of the disc than in the outer peripheral part thereof, so a uniform ion implantation is impossible.
The ion beam is deflected by a deflecting magnetic field externally established, to perform the radial scanning of the disc. This leads to the problem that the sectional shape and density distribution of the ion beam change subtly due to ununiformity in the deflecting magnetic field, the fringe effect at an end part of the magnetic field, etc., so a uniform ion implantation is impossible.